Setting

In the portlet edit form you can define which filters should appear in the portlet:

Criteria title - The title of criteria displayed in the portlet
Main index - Name of index which is used to group it ( it has to be in metadata too )
Value - The value of the index that the brain has to match to group

And additional values:

Expand list with unique values - Check box to enable the sub filtering
Sub index - Name of index which is used for listing unique groups dynamically

You can add as many criteria as you like

Hint: To enable the portlet at the search template, just add it to your site root
and remove the line <metal:block fill-slot=”column_two_slot” /> to enable portlets at all.

TODO

How to determine when display the portlet and when hide - fix the available property